Chapter I: Preface

The Christmas Lectures at the Royal Institution are, by a time-honoured custom, invariably addressed to a “juvenile audience.” This term, however, has always been held to be an elastic one, and to include those who are young in spirit as well as those who are young in years. The conditions, therefore, necessarily impose on the Lecturer the duty of treating some subject in such a manner that, whilst not beyond the reach of youthful minds, it may yet possess some elements of interest for those of maturer years. A subject which admits of abundant experimental illustrations is accordingly, on these occasions, a popular one, particularly if it has a bearing upon topics then attracting public attention. The progress of practical invention or discovery often removes at one stroke some fact or principle out of the region of purely scientific investigation, and places it within the purview of the popular mind. A demand then arises for explanations which shall dovetail it on to the ordinary experiences of life. The practical use of æther waves in wireless telegraphy has thus made the subject of waves in general an interesting one. Hence, when permitted the privilege, for a second time, of addressing Christmas audiences in the Royal Institution, the author ventured to indulge the hope that an experimental treatment of the subject of Waves and Ripples in various media would not be wanting in interest. Although such lectures, when reproduced in print, are destitute of the attractions furnished by successful experiments, yet, in response to the wish of many correspondents, they have been committed to writing, in the hope that the explanations given may still be useful to a circle of readers. The author trusts that the attempt to make the operations of visible waves a key to a comprehension of some of the effects produced by waves of an invisible kind may not be altogether without success, and that those who find some of the imperfect expositions in this little book in any degree helpful may thereby be impelled to study the facts more closely from that “open page of Nature” which lies ever unfolded for the instruction of those who have the patience and power to read it aright.

J. A. F.

UNIVERSITY COLLEGE,
LONDON, 1902.

CONTENTS.

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CHAPTER I.

WATER WAVES AND WATER RIPPLES.

PAGE

A visit to the seaside—What is a wave?—Wave-motion on
water—Definition of a wave—Sea waves—Various forms of
wave-motion—Wave length, velocity, and frequency—Atlantic
waves—Rules for speed of sea waves—Illustrations of
wave-motion—A stone falling on water—Production of a
wave-train—Wave-energy—Conditions for the production
of wave-motion—Distinction between wave-velocity and
wave-train velocity—Why a wave breaks—Waves in canals—Rule
for speed of a canal wave—Falling bodies—A “bore”—Tidal
waves—Ripples—Distinction between waves and ripples—Surface
tension on liquids—A needle floating on water—Experimental
production of ripples—Reflection and refraction of ripples
and waves—Interference of waves and ripples—Photography of
waves and ripples 1

CHAPTER II.

WAVES AND RIPPLES MADE BY SHIPS.

Ship-waves—The viscosity of liquids—How it is
demonstrated—Rotational and irrotational motion in fluids—Eddies
and whirls—Smoke rings—Vortex motion—Professor Hele-Shaw’s
experiments—Irrotational or stream-line motion in water—The
motion of water round a ship—The motion of water along a
pipe—Flow in uniform pipes and non-uniform pipes—Relation
between fluid velocity and pressure—Skin resistance and
wave-making resistance—The movement of a fish—Motion through
a perfect fluid—The waves made by moving objects—Waves made
by ducks and swans—Echelon waves—Ship bow waves—The form
of ship-waves—Mr. Froude’s experiments—Ship-models and
experimental tanks—How a ship is designed—Froude’s laws—Testing
ship-models—The design of a racing-yacht—Comparison of British
and American yachts—The Cup race—Scott Russell’s experiments on
canal-boats 57

CHAPTER III.

WAVES AND RIPPLES IN THE AIR.

Air necessary for the production of sound—A sounding body is
in vibration—Harmonic motion—The difference between noise and
music—The nature of an air wave—The physical qualities of
air—Longitudinal or compressional waves—Wave-models to illustrate
the nature of sound waves—Quality of a sound—Velocity of an air
wave—An illustration on a gigantic scale—The voice of a volcano
heard round the world—The effect of temperature on air-wave
velocity—Comparison of theory and experiment—Circumstances
affecting distance at which sounds can be heard—Funeral
guns—Fog-signals and sirens—Effect of wind and density—Sensitive
flames as sound-detectors—Inaudible sounds—The reflection and
refraction of sound waves—A sound-lens and sound-prism—The
interference of sounds—Two sounds producing silence—The
phonograph—A soap-bubble film set in vibration by air waves 103

CHAPTER IV.

SOUND AND MUSIC.

The difference between sounds and musical tones—The natural
period of vibration of an elastic body—The effect of accumulated
impulses—Free and forced vibrations—Breaking down a bridge with
a pea-shooter—The vibration of a stretched string—Stationary
waves—A string vibrating in segments—Acoustic resonance—Nodes
and anti-nodes—The musical scale or gamut—Musical intervals—The
natural gamuts and the scale of equal temperament—Concords and
discords—Musical beats—Helmholtz’s theory of discords—Musical
instruments—Pipes—Strings and plates—A pan-pipe—An
organ-pipe—Open and closed organ-pipes—The distribution of
air pressure and velocity in a sounding organ-pipe—Singing
flames—Stringed instruments—The violin—The Stroh violin—The
structure of the ear—The ear a wonderful air-wave detector
and analyzer 147

CHAPTER V.

ELECTRIC OSCILLATIONS AND ELECTRIC WAVES.

The conception of an æther—The phenomena of light require the
assumption of an æther—The velocity of light—Interference
of light—Two rays of light can produce darkness—An
electric current—The phenomena of electricity require the
assumption of an electro-magnetic medium—Properties and
powers of an electric current—Alternating and continuous
electric currents—Electromotive force and electric
strain—A Leyden jar—The oscillatory discharge of a
condenser—Oscillatory sparks—Transformation of electric
oscillations—Hertz oscillator—Production of a wave of
electric displacement—Detection of electric waves—Metallic
filings detectors—The coherer—Inductance and capacity of
circuits—Electro-static and electro-magnetic energy—An
induction coil—Electric oscillations give rise to electric
waves—The electron theory of electricity 185